KABUL -- A powerful car bomb exploded early Tuesday within a few feet of a passing military convoy on the western edge of Kabul, killing at least a dozen Afghan civilians and six foreign troops, including five Americans, U.S. military and Afghan officials said.

The Islamist Taliban movement asserted responsibility for the blast, which it said was carried out by a suicide bomber driving a vehicle packed with 1,650 pounds of explosives.

The blast overturned a heavy military truck and gouged an enormous crater in a street near the ruined Darulaman Palace, once home to Afghanistan's royal family, witnesses said. Five vehicles in the convoy were heavily damaged, along with more than a dozen civilian cars and a bus.

It was the first major bombing in the Afghan capital since February, and one of the deadliest in recent memory for troops of the International Security Assistance Force (ISAF).

A U.S. military spokesman said five of the troops killed in the blast were Americans, and the Canadian government said one of its service members also died.

The Afghan Interior Ministry reported that at least 12 Afghan civilians were killed and 47 wounded in the bombing, which occurred about 8 a.m. as people were on their way to work. Many of the dead civilians had been waiting at a nearby bus stop on the busy Kabul road that runs past the former palace and government ministries.

The attack was condemned by ISAF officials and by Afghan President Hamid Karzai, who only recently returned to Kabul from a visit to Washington.

In Washington, deputy White House press secretary Bill Burton said, "While our troops are fighting for a better future for the Afghan people, the Taliban offers only destruction, and they have so little respect for humanity that they would murder Afghan civilians waiting for a bus."

NATO Secretary General Anders Fogh Rasmussen said the attack would not deter the mission to "protect the Afghan people and strengthen Afghanistan's ability to resist terrorism."

The bombing occurred during preparations for a "peace jirga," or conference, organized by Karzai to try to reconcile the country's warring factions and pave the way for an eventual withdrawal of international troops.

The site of the blast, a traffic circle on a main road, became a scene of devastating carnage, with scattered body parts and victims missing limbs or parts of their heads, witnesses said.

Ali Ahmed Ibrahimkhel, a shopkeeper, joined others in collecting pieces of flesh and putting them into plastic bags.

Witnesses told local television reporters that the bomb was inside a minivan that drove up to the U.S.-led military convoy as it traveled along the road. The bombing occurred a week after a Taliban group issued a statement warning of "ambushes, detonations of explosive devices, assassinations of government officials, suicide bombings and detainment of foreign invaders."

More than 200 ISAF troops have been killed in Afghanistan this year.

Baryalai Sherzai, who owns a gym in Kabul, said he was driving behind the bomber's vehicle when he and his mother, a passenger, heard a huge explosion. Everything went dark, the windows of the car shattered and his mother cut her head, he said.

"I don't know if my mother is dead or alive," Sherzai, 27, said outside a hospital where she had been taken for treatment.

Special correspondent Javed Hamdard in Kabul and correspondent Joshua Partlow in Kandahar contributed to this report.